Why are some nuclei stable while others decay, and how can we predict the type of decay from a nucleus's composition?
The relationship between the numbers of neutrons and protons in stable and unstable nuclei, the N against Z graph, and predicting the mode of decay including alpha, beta-minus, beta-plus and gamma emission.
A focused answer to AQA A-Level Physics 3.8.1.5, covering the relationship between neutron and proton numbers in stable nuclei, the N against Z graph, and predicting alpha, beta-minus, beta-plus and gamma decay from a nucleus's position on the graph.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
AQA specification point 3.8.1.5 wants you to use the graph of neutron number against proton number to describe stable and unstable nuclei and to predict the type of decay a nucleus will undergo to become more stable.
The band of stability
The reason is the balance of two forces inside the nucleus: the short-range strong nuclear force, which attracts all nucleons, and the long-range electrostatic repulsion, which acts only between protons. As more protons are added, more neutrons are needed to provide extra strong-force attraction (with no extra repulsion) to keep the nucleus bound.
Predicting the mode of decay
On the graph, beta-minus moves a nuclide diagonally down and to the right, beta-plus moves it up and to the left, and alpha moves it diagonally down and to the left, each step taking it closer to the band.
Gamma emission
The decay equations
Try this
Q1. A nucleus lies above the band of stability. State the type of decay it will undergo. [1 mark]
- Cue. Beta-minus decay (it has too many neutrons).
Q2. Why do heavy nuclei need proportionally more neutrons than protons to be stable? [2 marks]
- Cue. More neutrons add to the strong nuclear attraction without adding electrostatic repulsion, balancing the growing proton-proton repulsion.
Q3. State the effect of gamma emission on the nucleon number of a nucleus. [1 mark]
- Cue. No change.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksA nucleus of sodium-24 () lies above the band of stability. State the type of decay it undergoes, write the decay equation including any leptons emitted, and explain how the decay moves it towards stability.Show worked answer →
Lying above the band means it has too many neutrons, so it decays by beta-minus emission. A neutron becomes a proton, emitting an electron and an antineutrino.
The equation is .
This reduces the neutron number by one and raises the proton number by one, moving the nuclide down and to the right on the N against Z graph, towards the band of stability.
Markers reward identifying beta-minus, a correctly balanced equation including the antineutrino, and explaining the move towards the band.
AQA 20213 marksExplain why gamma emission often follows alpha or beta decay, and state how gamma emission affects the proton number and nucleon number of a nucleus.Show worked answer →
After an alpha or beta decay the daughter nucleus is often left in an excited (higher energy) state. It loses this excess energy by emitting a high-energy gamma photon, falling to a lower energy state.
Gamma emission removes only energy, not nucleons or charge, so it leaves both the proton number and the nucleon number unchanged.
Markers reward the excited daughter nucleus, energy released as a gamma photon, and no change to or .
Related dot points
- Radioactive decay as a random process, the decay constant, the activity of a source, the exponential decay law, half-life and applications such as radioactive dating.
A focused answer to AQA A-Level Physics 3.8.1.4, covering radioactive decay as a random process, the decay constant, activity, the exponential decay law, half-life and its link to the decay constant, and radioactive dating.
- The nature, penetration, ionising power and range of alpha, beta and gamma radiation, the inverse square law for gamma, background radiation and the uses and hazards of radiation.
A focused answer to AQA A-Level Physics 3.8.1.2 and 3.8.1.3, covering the nature, penetration, range and ionising power of alpha, beta and gamma radiation, background radiation, the inverse square law for gamma rays and the safe uses of radiation.
- Mass and energy equivalence, mass defect and binding energy, the binding energy per nucleon curve, and the energy released in fission and fusion.
A focused answer to AQA A-Level Physics 3.8.1.7 and 3.8.1.8, covering mass and energy equivalence, mass defect, binding energy and binding energy per nucleon, the binding energy curve, and the energy released in nuclear fission and fusion.
- Estimating nuclear radius from closest approach of alpha particles and from electron diffraction, the dependence of radius on nucleon number, and the constancy of nuclear density.
A focused answer to AQA A-Level Physics 3.8.1.6, covering estimates of nuclear radius from alpha particle closest approach and electron diffraction, the relationship R proportional to the cube root of A, and the constant density of nuclear matter.
- The Rutherford alpha particle scattering experiment, the observations and conclusions, and how they led to the nuclear model of the atom.
A focused answer to AQA A-Level Physics 3.8.1.1, covering the Rutherford and Geiger and Marsden alpha scattering experiment, the key observations and the conclusions they support about the nuclear model of the atom.
Sources & how we know this
- AQA A-level Physics (7408) specification — AQA (2017)